The atomic force microscope (AFM) is one of the most versatile tools in the exploration and manipulation of micrometric and nanometric-sized systems. Since its invention in the 1980's, this technique has continued improving to the point where it has conquered practically all working environments—from systems that operate under ultra-high vacuum (UHV) conditions to those that are immersed in a liquid medium via, of course, the customary atmospheric conditions in which we ourselves live.
The AFM is based on a micro-lever that acts as a probe. This micro-lever, arranged in a cantilevered manner, in other words with one end fixed and the other one free, has a very sharp tip at its free end. The operation of this tool is based on making the tip of the micro-cantilever interact with the sample or system being studied. Said interaction causes changes in some observable characteristics of the micro-cantilever, these being finally translated into changes in the position of the same. A fundamental part of the AFM is therefore the micro-cantilever position detection system. The measurement of the micro-cantilever's position is generally taken by means of an optical system comprising a laser light beam suitably focused on the free end of the micro-cantilever. The changes in position of said micro-cantilever then produce changes in the direction of the reflected laser light beam, which is picked up by a photodiode.
The contact working mode involves directly supporting the micro-cantilever on the sample being studied, as if it were a profilometer, bringing it into proximity with and drawing it away from the sample, in order to keep the position of the micro-cantilever constant during the relative displacement of the tip on the sample. This method of working is obviously highly invasive due to the direct tip-sample contact, which means that it can only be used on relatively hard, rigid samples in which the forces applied do not constitute a problem.
In order to avoid the disadvantages of the contact mode, the idea emerged of eliminating direct contact between the tip and the sample. The method of avoiding this contact involves making the micro-cantilever oscillate with a sinusoidal movement of one or more simultaneous frequencies.
Said movement is characterized by its oscillation amplitude and frequency, which can be determined by measuring the changes in position of the micro-cantilever over time. Without there being any need to make direct contact in this case, the tip-sample interaction alters the oscillation frequency and amplitude.
In the publication by Albrecht, T. R., et al, “Frequency-Modulation Detection Using High-Q Cantilevers for Enhanced Force Microscope Sensitivity”, Journal of Applied Physics, 1991, 69(2): p 668-673, frequency is taken as the control condition while maintaining a fixed value for the oscillation amplitude. This method (referred to as Frequency Modulation or FM) is far more sophisticated than the previous one, requiring a far more powerful control electronics, as well as extensive experience on the part of the user in order to operate it. Making significant changes in the operating conditions allows work to be carried out in different environments.
The main problem associated with the FM method is the possible loss of control of the AFM due to changes in sign in the interaction. The tip-sample interaction curve in FIG. 1 (canonical curve when work is carried out in air or in a vacuum) illustrates the reason for this loss of control. As we can see in FIGS. 2a and 2b, there are two regions differentiated by a change of sign in the slope which reflects both increasing and reducing behaviour in the change in frequency as the tip approaches the sample. The problem originates in the need to indicate to the feedback system an agreement for altering the manipulated variable. For example, this agreement may be as follows “an increase in the controlled variable is corrected with an increase in the manipulated variable”. In this way, the feedback system will work correctly in the region of the curve where this behaviour is established, but will lose control if for some reason it is situated in another region. Due to disturbances (mechanical noise, electrical noise, etc.) it is very common for the domain that does not satisfy the control condition to be entered, thereby destabilizing the microscope. The above problem does not mean that the microscope only has access to the domain that does not satisfy this agreement simply by multiplying the signal from the controlled variable by −1, bearing in mind that in this case the other domain will remain out of the control of the feedback loop, without having to state the interaction domain (attractive or repellent) in which work is to be carried out before putting it into operation. Given an agreement for our feedback system, we are able to work using the repellent domain. In liquid media, unlike in a vacuum or in air, the attractive interaction is very small or virtually negligible, as a result of the screening of the van der Waals forces that occur when the micro-cantilever is completely surrounded by molecules of the liquid medium. However, the screening of van der Waals forces in liquids does not mean that long-range interactions cannot appear in these conditions. It is fairly common that when a surface is immersed in a liquid, it is often charged due to the presence of functional ionized groups on the surface and/or the adsorption of ions present in the liquid solution. As a result, this charge present on the surface electrically attracts counterions in the solution, giving rise to the formation of a double electrical layer. The tip of the micro-cantilever in liquids may also appear surrounded by a double electrical layer. The interaction of both double-layer structures, as the tip-sample distance diminishes, results in a local electrical force which may complicate interpretation hugely.
On the other hand, the considerable reduction in the attractive range in liquids is not always certain, since on a multitude of occasions attractive interactions appear between the tip and the sample as a result of the capture of unwanted material by the tip, coming from the sample itself or from the liquid medium. Hence, there may be a destabilization of the feedback loop that controls the tip-sample distance.